1. Field of the Invention
The present invention relates generally to a frequency controlled signal generating circuit, and is directed more particularly to a frequency controlled signal generating circuit suitable for use in an APC (automatic phase control) circuit provided in the reproducing system of a VTR (video tape recorder).
2. Description of the Prior Art
In a VTR, the color carrier (chroma) signal is first converted into a low frequency signal and then superimposed on a luminance signal which is, for example, frequency-modulated and thereafter the resulting combined signal is recorded. In a VTR reproducing system there is provided a frequency reconverting circuit, which restores the chroma signal to its original frequency by means of a carrier signal.
The carrier signal for the frequency reconversion is provided by an APC circuit 10 shown in FIG. 1. In the figure, a terminal 1 is supplied with a burst signal S.sub.B (3.58 MHz) which has been separated from a reproduced chroma signal after frequency-reconversion of the latter. The burst signal S.sub.B together with a reference signal S.sub.R (3.58 MHz) derived from a reference oscillator 2 are fed to a phase comparator 3 to be phase-compared therein. The resulting output from the phase comparator 3 is fed to a low-pass filter 4, where output constitutes a control voltage V.sub.C applied to a variable frequency oscillator 5. This oscillator 5 generates a signal with a central frequency of 3.58 MHz plus a jitter component in response to the control voltage V.sub.C.
The signal from the oscillator 5 and an oscillating signal S.sub.L (688 KHz in this example) from AFC circuit 8 are both fed to a frequency converting circuit 6 in which a carrier signal S.sub.C having a predetermined frequency (4.27 MHz) is generated. Signal S.sub.c is the carrier signal supplied through a terminal 7 to a frequency-reconverting circuit provided in the color processing circuit of the reproducing side of the VTR for restoring the color carrier from the frequency (688 KHz) with which it is recorded to its standard frequency (3.58 MHz).
With the APC circuit 10 constructed as above, quartz oscillating elements 2a and 5a are used in both the reference oscillator 2 and the variable oscillator 5. If a quartz oscillating element 5a is used as the oscillating element of variable oscillator 5, the control sensitivity of the oscillator is relatively low. Therefore, it becomes difficult to rapidly restore a color signal to its original hue by correcting a large variation of the phase of the reproduced color signal.
On the contrary, a relaxation oscillator not using a quartz element is generally high in control sensitivity and can vary the frequency up to about several tens of KHz. Therefore, such a relaxation oscillator may advantageously be used as the variable oscillator 5. In this situation, however, the problem of frequency drift may occur. Owing to variations in the capacitors and the resistors incorporated in the relaxation oscillator due to their temperature characteristics and so on, the oscillation frequency may fluctuate as much as several hundreds of KHz. An oscillator whose frequency fluctuates in such a fashion is unsuitable for use as variable oscillator 5.